1. Field of the Invention
The present invention relates to a structure with a through hole, comprised of a silicon (Si) semiconductor substrate and other elements, and a production method thereof and, more particularly, to a structure suitably applied to thermal recording heads, ink jet recording heads, etc., used in printers and other devices, a production method thereof, and a liquid discharge head and apparatus having the structure.
2. Related Background Art
Structures with through hole(s) are used in various fields. For example, a structure with through hole(s) comprised of a silicon semiconductor substrate and other elements is used in ink jet recording heads which are used in ink jet printers and other devices and adapted to discharge ink to implement recording. The following will describe a structure with a through hole, using an example of an ink jet recording head configured to discharge ink by thermal energy.
The ink jet recording head utilizing thermal energy is configured to impart thermal energy generated by a heating resistive element (heater), to a liquid to cause a bubbling phenomenon selectively in the liquid and discharge an ink droplet from each discharge opening by the bubbling energy. In the ink jet recording head of this type, in order to increase the recording density (resolution), a number of fine heating resistors are arranged on the silicon semiconductor substrate, discharge openings are provided for the respective heating resistors so as to face the heating resistors, and drive and peripheral circuits for driving the heating resistors are also provided on the silicon semiconductor substrate.
FIG. 8 is a sectional view showing a configuration of such an ink jet recording head.
As shown in FIG. 8, the ink jet recording head is constructed in a structure wherein a field oxide film (LOCOS oxide film) 101, a BPSG (boro-phospho silicate glass) layer 102 deposited by atmospheric CVD (chemical vapor deposition), and a silicon oxide film 103 deposited by plasma CVD are stacked on one principal surface of silicon substrate 100, heating resistors (heaters) 110 are formed on the silicon oxide film 103, and a discharge opening 140 is provided so as to face each heating resistor 110. In the drawing only one heating resistor 110 and one discharge opening 140 are depicted, but in fact, several hundred heating resistors and discharge openings are arranged in one ink jet recording head. These heating resistors are arranged at predetermined intervals (e.g., 40 μm) in the direction normal to the plane of the drawing on the single silicon substrate 100.
In order to protect the heating resistors 110 and other elements, a silicon nitride film 104 is formed as a passivation layer by plasma CVD, over the whole of the aforementioned principal surface of the silicon substrate 100 including the regions on the heating resistors 110. In portions corresponding to the heating resistors 110 along the surface of the silicon nitride film 104, tantalum (Ta) films 105 are formed as anti-cavitation layers in order to prevent deterioration of the silicon nitride film 104 due to the cavitation phenomenon caused by bubbles generated in the ink. The principal surface of the silicon substrate 100 other than the principal surface on which the heating resistors 110 are formed, is covered by a thermal oxide film 106.
The discharge openings 140 are bored in a covering resin layer 130 provided so as to cover the first aforementioned principal surface of the silicon substrate 100. A space is formed between the covering resin layer 130 and, the silicon nitride film 104 and tantalum film 105, and this space is a space filled with a liquid (ink) to be discharged from the discharge opening 140. This space will be called a liquid chamber 150.
In the ink jet recording head having the structure described above, when each heating resistor 110 is energized to generate heat, the heat generates a bubble in the discharge liquid in the liquid chamber 150 and an action force of the bubble thus generated discharges a liquid droplet from the discharge opening 140. In order to implement continuous recording, it is necessary to replenish the liquid chamber 150 with the discharge liquid (ink) by the amount of liquid contained in the liquid droplets discharged from the discharge opening 140. However, the discharge openings 140 are located in the proximity of a recording medium, such as paper or the like, and the gap is also set to be small between the discharge openings 140 and the heating resistors 110. It is thus difficult to supply the discharge liquid from the side of the silicon substrate 100 where the heating resistors 110 are formed, into the liquid chambers 150. For this reason, as illustrated, supply openings 120 are formed through the silicon substrate 100 and the discharge liquid is allowed to flow through the supply openings 120 in the direction indicated by the arrow in the drawing, whereby the discharge liquid is supplied into the liquid chambers 150. The supply openings 120 are formed by selective etching of the silicon substrate 100.
Meanwhile, the silicon substrate 100 normally has a thickness of several hundred μm. If the silicon substrate 100 were etched from the principal surface where the heating resistors 110 are formed, in order to form the supply openings 120 by etching, the etching process would take a long time even under setting of selective etching conditions and there would inevitably occur damage to each of the layers formed on the principal surface and to the heating resistors 110. The supply openings 120 are thus formed by etching of the silicon substrate 100 from the principal surface other than that on which the heating resistors 110 are formed. In that case, if an etchant flows to the side where the heating resistors 110 are formed, upon penetration of the supply openings 120, it can cause damage to the heating resistors 110 and/or each of the other layers. Therefore, layers as etching stoppers are preliminarily provided at positions intended for formation of the supply openings 120 on the principal surface of the silicon substrate 100 on which the heating resistors 110 are formed, whereby the etchant is prevented from flowing to the side where the heating resistors 110 are formed.
In the example shown in FIG. 8, the field oxide film 101, BPSG layer 102, and silicon oxide film 103 are not provided in the regions where the supply openings 120 are formed, and, instead thereof, silicon nitride films 107 formed by reduced pressure CVD are provided. The silicon nitride films 107 are patterned so as to be located only in the forming and surrounding regions of the supply openings 120, and ends thereof are formed so as to be interposed between the field oxide film 101 and the BPSG film 102. In the forming region of each supply opening 120, the silicon nitride film 107 is directly deposited on a thin oxide film 108 on the surface of the silicon substrate 100. The silicon nitride film 104 deposited by plasma CVD is also formed on the silicon nitride films 107 by reduced pressure CVD.
During the final stage of etching, the silicon nitride film 107 is exposed in the bottom of each supply opening 120 formed, as described later. If in this stage the silicon nitride film 107 and the silicon nitride film 104 crack or peel off from the silicon substrate 100, the etchant will leak to the side of the heating resistors 110, which is not preferred. For this reason, as also described in Japanese Patent Application Laid-Open No. 10-181032 (counterpart of U.S. Pat. No. 6,143,190), the silicon nitride film 107 is formed by reduced pressure CVD, so that the internal stress of the silicon nitride film 107 becomes a tensile stress, which can prevent the occurrence of peeling or the like.
Now, the structure of the heating resistor 110 will be described. FIG. 9A is a schematic perspective view illustrating the structure of the heating resistor (heater), and FIG. 9B is a circuit diagram showing the part including the heating resistor and a switching device for driving it.
The heating resistor 110 is made in such a manner that a resistive layer 111 made of an electrically resistive material, such as tantalum silicon nitride (TaSiN) or the like, and an aluminum (Al) layer 112 as an electrode layer are formed in the same pattern, and a part of the aluminum layer 112 is removed so that only the resistive layer 111 remains in that part of the heating resistor 110. The part where only the resistive layer 111 exists serves as a heat generating portion upon supply of electricity (heating surface H). In the example illustrated, the resistive layer 111 and aluminum layer 112 are deposited in the order named on the silicon oxide layer 103; thereafter, unnecessary portions of the two layers are first removed by etching so as to leave a U-shaped pattern, and then only the aluminum layer 112 is further removed in the part becoming the heating portion H, thereby completing the heating resistor 110. Thereafter, the entire heating resistor 110 is covered by the silicon nitride film 104 as a passivation layer.
A method of producing the ink jet recording head as described above will be described next. In order to simplify the description, the following discussion will exclude description of the thermal oxide film 106 formed on the side of the silicon substrate 100 other than the side on which the heating resistors 110 are formed, and FIGS. 10A to 10D and 11A to 11C show only the structure of a supply opening 120 (the forming position thereof) and its surroundings.
The production method of the ink jet recording head using the silicon substrate with through holes is described, for example, in Japanese Patent Application Laid-Open No. 10-181032. First, as shown in FIG. 10A, the field oxide film 101 is selectively formed, for example, in a thickness of about 700 nm, on one principal surface of the silicon substrate 100. A thin oxide film 108 is formed in the region where no field oxide film 101 is formed. Next, as shown in FIG. 10B, the oxide film 108 is removed at a portion corresponding to the forming position of the supply opening 120 to expose the silicon surface and, as shown in FIG. 10C, a polysilicon layer 121 to become a sacrificial layer is further selectively formed at the exposed position of the silicon surface, for example, in a thickness of 200 to 500 nm. At this time, the silicon surface without the oxide film 108 completely surrounds the polysilicon layer 121. Thereafter, as shown in FIG. 10D, the silicon nitride film 107 is selectively formed at and around the forming position of the supply opening 120 by reduced pressure CVD. The thickness of the silicon nitride film 107 is, for example, approximately 200 to 300 nm.
Next, as shown in FIG. 11A, the BPSG layer 102 is formed, for example, in a thickness of 700 nm over the entire surface of the silicon nitride film 107 and the field oxide film 101 by atmospheric CVD, and the silicon oxide film 103 is then formed, for example, in a thickness of 1.4 μm over the entire surface of the BPSG layer 102 by plasma CVD. The surface of the silicon oxide film 103 is almost flat. Then, as shown in FIG. 11B, the silicon oxide film 103 and the BPSG layer 102 are selectively removed in a region that corresponds to the position where the supply opening 120 is to be formed but that is a little larger than the to-be-formed supply opening 120. At this time, the ends of the removed part are located at positions where they are placed on the silicon nitride film 107, and the field oxide film 101 also exists below it.
Subsequently, the resistive layer 111 and aluminum layer 112 are formed, these are then patterned in the U-shaped pattern as described above, and the aluminum layer 112 is further selectively removed from the position to become the heating portion, thereby forming the heating resistor 110 on the silicon oxide film 103. Thereafter, as shown in FIG. 1C, the silicon nitride film 104 to become a passivation layer is formed, for example, in a thickness of 800 nm over the entire surface, the tantalum film 105 as an anti-cavitation layer is selectively formed, and the silicon substrate 100 at the supply opening forming position and the polysilicon layer 121 as a sacrificial layer are removed by anisotropic etching from the side where no heating resistor 110 is formed on the silicon substrate 100 (the lower side in the drawing), thereby forming the supply opening 120. At this time, the silicon nitride film 107 lined with the silicon nitride film 104 is exposed as a so-called membrane in the bottom part of the supply opening 120. In the final stage of the etching, only this membrane serves to prevent the etchant from entering the heating resistor 110 side, so that keeping the membrane from cracking or peeling off contributes to great improvement in the yield of recording heads.
Finally, the silicon nitride film 107 and the silicon nitride film 104 are removed from the region located at the bottom surface of the supply opening 120 by dry etching using a fluorine base or oxygen base gas. This completes the substrate for the recording head having the supply opening 120 for supply of ink or the like as a through hole. Thereafter, the covering resin layer 130 and the discharge opening 140 are formed by known methods.
Among the above steps, the patterning steps necessary for formation of the supply opening 120 (only those necessitating photomasks) include the step of removing part of the oxide film 108 as shown in FIG. 10B, the step of selectively providing the polysilicon layer 121 as shown in FIG. 10C, the step of selectively providing the silicon nitride film 107 as shown in FIG. 10D, the step of removing the BPSG layer 102 and the silicon oxide layer 103 by etching corresponding to the position of the supply opening 120 as shown in FIG. 11B, and the step of forming the supply opening 120 by etching of the silicon substrate 100 as shown in FIG. 1C.
On the other hand, the heating resistor 110 is connected at one end, for example, to a power supply VH of about +30 V and at the other end to a drain D of a MOS field effect transistor Ml as a switching device for driving, as shown in FIG. 9B. A source S of the transistor M1 is grounded and drive pulses are applied to a gate G of the transistor to drive the resistor. In the case where the drive circuit, including the transistor Ml, and other peripheral circuits are formed on the silicon substrate 100, the BPSG layer 102 and the silicon oxide film 103 are formed so as to serve as interlayer dielectrics and the silicon nitride film 104 is formed so as to serve as a passivation layer. The field oxide film 101 is used for device isolation in the regions where the drive and peripheral circuits are formed.
In the conventional structure, the reason why the silicon nitride film 107 deposited by reduced pressure CVD is intentionally used as the membrane serving as an etching stopper during the formation of the supply opening 120 by etching, is that the internal stress of this film is a tensile stress. In contrast, the internal stress of the silicon oxide film 103 deposited by plasma CVD is a compressive stress. It has been believed heretofore that it was necessary to maintain the tension of the membrane through use of a film with a tensile stress as the membrane in order to avoid cracking or peeling off of the membrane during etching. For this reason, the silicon nitride film 107 deposited by reduced pressure CVD has been used. Namely, it has been believed that the problem of cracking or peeling was unavoidable in the case of a film with a compressive stress.
In the case of the conventional production method of ink jet recording head described above, even if the method is arranged to simultaneously perform the step of providing the supply opening as a through hole in the silicon substrate and the step of forming the heating resistor, drive circuit, and peripheral circuits on the silicon substrate, the method requires at least five photomasks associated with only the step of providing the supply opening, and the total number of photomasks used is 17 or 18, including processing of other portions not here described. The steps are thus complex. Particularly, the silicon nitride film with the tensile stress (the silicon nitride film deposited by reduced pressure CVD in the above example) needs to be provided as a membrane by patterning and the silicon oxide film has to be patterned near the forming position of the through hole, which poses a problem that the number of manufacturing steps is large.
Since the conventional silicon nitride layer serving as a passivation layer and as a membrane has a typical thickness of about 800 nm, the membrane region has a large compressive stress and the silicon nitride film having a tensile stress is needed for relaxation thereof. On the other hand, in the case of silicon nitride of this thickness, the heat generated by the heating resistor is transferred through this silicon nitride film to the discharge liquid, so that the utilization efficiency of the heat is not satisfactory, thus posing a problem that the frequency is limited in the case of executing repetitive discharges.